Field of the Invention
The present invention relates to an antenna, a wireless communication device, and an electronic device.
Description of the Related Art
In recent years, in imaging devices such as digital cameras, and other electronic devices, for example, a wireless communication device having a wireless function is mounted. An electronic device in which a wireless communication device is mounted is able to wirelessly transmit a signal such as a captured image to another camera, a personal computer (PC), or the like by using a wireless Local Area Network (LAN), Bluetooth (registered trademark), or the like. Radio waves of 2.4 GHz, 5 GHz, and the like are used in a wireless communication by a wireless LAN, Bluetooth (registered trademark), and the like. A wireless communication device may be built into an electronic device or may be installed as an external option in an imaging device such as a digital single-lens reflex camera, for example. With installation of such an option in an imaging device, it is possible to wirelessly communicate with another device at a distance of 100 meters or more, for example.
With respect to this type of wireless communication devices, there is a concern that, when a high power electromagnetic wave emitted from the antenna enters a human body and the energy of the electromagnetic wave is absorbed in the human body, a local rise in temperature may occur in the human body. It is indicated that such a local rise in temperature of a human body is likely to increase a risk of development of cataract or the like, for example. Therefore, in each country, an amount of absorption of electromagnetic waves by a human body is evaluated as Specific Absorption Ratio (SAR), and a regulation value thereof is defined.
In respect to a technique for suppressing the SAR below a regulation value, “Development of Reduction Technology of the Electromagnetic Wave Exposure for Biologic Body”, Atsushi Igarashi et al., The Institute of Electronics, Information and Communication Engineers 2008, Tokyo branch student council, a meeting for presenting research papers, Feb. 28, 2009, p. 67, B-4 (hereafter, referred to as “Igarashi et al.”) discloses that the SAR is inversely proportional to the square of the distance between an antenna and a phantom.
Further, Japanese Patent Application Laid-open No. 2005-184703 discloses that, for improving the SAR, a part of an electromagnetic wave emitted from the antenna element is shielded by a symmetrically structured ground plane adapted to a symmetrically structured antenna element.
According to Igarashi et al., the SAR value can be reduced by putting an antenna away from a human body. In order to put an antenna away from a human body, however, it is necessary to secure a larger space between an antenna and an outer casing of an electronic device accommodating the antenna. This results in an increased external size of the electronic device and it is therefore difficult to reduce the size or the thickness of the electronic device including an antenna.